Generally, in the magneto-optic recording process, a magnetic thin film having a perpendicular magnetic easy axis is used as a binary digital information storing system by magnetizing upwardly or downwardly.
Information recording is carried out as shown in FIG. 1A. A magnetic field of about 300 Oe is externally applied in the polarizational angle of desired direction, and at the same time the medium is heated by laser beams up to the Curie temperature above which the medium becomes paramagnetic. The laser beams are then withdrawn to allow the scanned regions of the medium to be magnetized in the same polarity as the external magnetic field, thereby recording the information.
As shown in FIG. 1B, the reading of the recorded information is carried out by utilizing the Polar Kerr effect. That is, if rotational polarized light rays are scanned onto a vertically magnetized material, the reflected light rays are rotated around the polarizing direction of the incoming light rays. The magnitude of the rotational angle depends on the magnitude of magnetization, and the rotational direction is clockwise or counterclockwise in accordance with the magnetized direction. The light rays which are reflected from the upwardly and downwardly magnetized spots are detected, by a photodiode thereby the information is read as a "1"or a "0".
As shown in FIG. 1C, recorded information is erased by applying the external magnetic field in a direction opposite to that of the recording magnetic field with an irradiating laser beam. Such recording, erasing and re-writing can be repeated infinitely by reversing the applied magnetic field.
In the currently used magneto-optic recording media, a thin film consisting of a rare earth-transition metal alloy is employed. The rare earth elements include Tb, Gd, Nd and Dy, while the transition metals include Fe, Co, and the like. One of the most superior recording media a Tb-Fe-Co thin film Sputtering and evaporation methods are principally used in manufacturing the rare earth transition motal alloy films. In the case of sputtering, an alloy target may be employed, or as in Co-sputtering, the rare earth and the transition metals serve as plural targets with an irradiating laser beam.
In the rare earth-transition metal alloy recording media, oxidation readily occurs because the rare earths have a high affinity for oxygen. In order to prevent oxidation, and to increase the rotational angle by multiple reflection during information reading, the recording layer is sandwiched by a dielectric medium. Among the known materials for the protective layer are Si.sub.3 N.sub.4, AlN, and other nitrides.
Yet, in the above described conventional technique, the rare earth-transition metal alloy recording medium is still likely to oxidize because the rare earths have such great affinities for oxygen. Although dielectric layers are employed in order to prevent the oxidation, this protection is not always sufficient.
Further, the recording medium is sensitive to the manufacturing process, and the yield is only about 10%. A technique to overcome the above described problems has been published: "Magnetic Optical Properties of Co/Pd Superlative Thin Film", J. Appl. Phys., 67, p. 317 (1990).
According to the published technique, the magneto-optic recording medium includes a multi-layer film of Co/Pd. The thickness of the Co sub-layer is 1-5 and the Pd sub-layer is 6-15.ANG.,with the layers being alternately stacked repeatedly until the total thickness of the recording medium becomes 110-270.ANG..
The manufacturing method for the multi-layer thin film employs a vacuum chamber which accommodates a rotary table. The substrates are mounted upon the rotary table and a plurality of crucibles containing either Co or Pd are disposed below the rotary table.
In such an apparatus, the Co and Pd in the crucibles is deposited onto the substrate by applying the thermal evaporation deposition method, and then a second deposition is performed by rotating the rotary table so that Pd is deposited on the Co, and vice versa. The depositions are repeated to form a multi-layer film of Co/Pd.
However, in the above described manufacturing method, production is limited making continuous mass production difficult. Further, the thermal evaporation deposition method makes the manufacturing method unsuitable for the high melting point materials such as Pt, and makes it difficult to obtain a uniform deposition.